Ruby is the red variety of the corundum family which consists of alumina having a particular crystalline structure. The red coloration of ruby is due to the presence of chromium in the corundum.
Polycrystalline rubies exist commercially which are obtained by mixing alumina and chromium oxide and a small amount of MgO as a sintering additive, by subsequently forming the mixture and by sintering it under a hydrogen atmosphere or under a vacuum of at least 10−1 Torr. The polycrystalline rubies obtained by these routes have, however, unsatisfactory mechanical characteristics due to a low homogeneity of the microstructures and to a grain size that is much too high since it often exceeds 10 microns. Furthermore, their translucent appearance is not desirable in certain scenarios where it would be preferable to be able to use a bulk-colored and opaque ceramic of high toughness.
Japanese patent application No. JP 56-140070 relates to a process for manufacturing alumina-based reddish-purple ceramics. These ceramics are translucent, they contain, according to the sole example from this application, chromium oxide, yttrium oxide, lanthanum oxide and 0.7% of magnesium oxide and the preparation thereof comprises a step of sintering under a reducing atmosphere, in particular under a hydrogen atmosphere.
Japanese patent application no. JP 56-140071 also relates to a process for manufacturing alumina-based reddish-purple ceramics. These ceramics are also translucent, they contain chromium oxide, magnesium oxide, lanthanum oxide and yttrium oxide. The preparation thereof comprises sintering under a hydrogen atmosphere. This application does not include any exemplary embodiment.
Japanese patent application no. JP 04-193760 relates to colored alumina-based materials. These materials are translucent. Contrary to what the abstract of this patent application erroneously indicates, these materials may contain up to 2%, and not 5%, of chromium oxide, cobalt oxide, nickel oxide, vanadium oxide, manganese oxide, iron oxide or titanium oxide, and also at least one rare-earth oxide chosen from praseodymium, neodymium and erbium oxides. Magnesium oxide may also be added as an agent that inhibits the growth of particles during the sintering. However, the amount of this oxide to be added is not indicated. The preparation of these materials includes a sintering step which may take place in air, at a temperature between 1300 and 1800°. This high-temperature sintering step must necessarily be followed by hot isostatic pressing (HIP, also known as heat isostatic pressing) under a pressure between 500 and 2000 atmospheres and at a temperature of 1400° C. The examples from this patent application describe the preparation of light green, light pink, light blue, very light green and very light blue materials.
Thus, none of the prior art documents relates to the manufacture of an alumina-based ceramic which is opaque and has satisfactory mechanical properties.